Amplification of electric waves



Jan. 20, 1942. J. M. wl-:sr 2,270,364

Y AMPLIFIGATION OF ELECTRIC WAVES Filed April 26, 1940 JM WEST BV v ATZORNEV Patented `Jan. 20, 1942 AMPLIFICATION 0F ELECTRIC WAVES Julian M. West, Ridgewood, N.\J., assigner to Bell Telephone Laboratories,

Incorporated, New` York, N. Y., a corporation of New York Application April 26, 1940, Serial No. 331,728 13 claims.v (o1. 17e- 171) Vstage coupling circuits provided.

r The invention is especially adapted for the amplification of television signals, Where an amplifier is required thatfhas a substantially constant 4gain from a low audio frequencyapproximating direct current toa radio frequency of the order of several megacycles per second and that has a linear phase shift characteristic. Although it is with these and other stringent requirements in View that the present invention will be described,

`it, is to be understood that in at least its broader aspects `the invention is in vno manner limited to the amplification of television signals or to ampliliers adapted for such signals,

vOne of the simplest, idealized forms which a multistage television amplifier may take comrprises two space discharge devices with the anode of one connected, to the control grid of the other and with the respective cathodes'conn'ected together, and a coupling impedance comprising a pure resistance connectedacross the circuit from the anode mentioned to the cathode. In attempting to realize this idealized form in practice difficulty is encountered in devising a coupling impedance that is purely resistive or that otherwise hasthe same impedance throughout the frequency range.' The input or interelectrode ca'- pacitance of the second discharge device and the output capacitance of the first discharge device,

`for example, appear across the coupling circuit and thereby vtogether contribute a shunt reactance that disturbs the idealized non-reactive circuit condition. An additional shunt reactance is introduced by the stray capacitance of the blocking condenser Ywhich must be interposed in the anode-grid connection to protect the grid from the high biasing voltage applied tothe anode. Although this stray capacitance may be objectionably large in any type of amplifier, it tends to be especially troublesome in the case assumed, for the presence of television signal components of very low frequency and the desire to maintain the Series impedance at a low andpreferably denser be a large one despite the relatively large amount of stray capacitance associated with it.v

In so far as the objective of a coupling impedance constant throughout the frequency range is concerned, the effect of shunt capacitance can be minimized in the usual manner by constructing the coupling impedance as a constant-resistance artificial line of which the shunt capacitance is one of the lumped impedance elements. However, the presence of shunt capacitance remains as what appears today to be an inherent limitation on the fiat or uniform gain that can be attained with a signal band of given frequency width.

In accordance with a principal feature of the invention the limitations and disadvantages incident to the use of `a relatively large blocking condenser are substantially avoided, without relinquishing the advantages of animpedance coupling of constant resistance between stages, in a coupling circuit utilizing two blocking condensers, one of comparatively small capacitance traversed bythe higher frequenciesof the signaling band and the other of comparatively large capacitance adapted for and traversed by the lower `frequencies. The large stray capacitance necessarily associated with the larger condenser is effectively shunted -across only the portions of the coupling circuit that carry the lower frequencies and at these lower frequencies the shunt reactance due to the stray capacitance is negligible.

Another amplifier circuit condition that should obtain for highgain, assuming that the discharge devices are screen grid tubes, is substantial equality of the anode and screen'grid biasing potentials. In accordance with a feature of the present invention this condition is brought about by a connection between the two electrodes that with each other are usually connected in a common portion of the control grid and anode circuits. The self-biasing circuit, however, tends to introduce negative feedback and to reduce the gain of the amplifier at the lower end of the frequencyrange. This tendency requires special attention in television signal amplifiers for at the very low signaling frequencies involved the condenser imperfectly performs its function of bypassing the resistor. In accordance with another feature of the invention the tendency noted is counteracted by a like condenser and resistance which modify the characteristics of the coupling impedance.

The nature of the present invention and its various objects, features and advantages will appear more fully in the following detailed description of the illustrative embodiments shown in the accompanying drawing:

Fig. 1 shows schematically a two-stage `screen grid tube amplifier in accordance with the invention;

Fig. 2 represents a simplified modification of Fig. 1;

Fig. 3 shows the amplifier of Fig. 1 with compensation for the effect of self-biasing; and

Figs. 4-and 5 illustrate modifications ofFig. 3.

Referringk -now to thespecic embodiment of the invention illustratedin Fig. 1, there is shown schematically the first two stages of an amplier adapted to amplify with high and uniform gain a band ofsignaling frequencies delivered to it from source I. Assuming that the signals from the-source are televisionsignals, the frequencies involved may range, for specific example, from forty .cycles .per second to four megacycles per second. The two discharge devices V1 and V2 comprising the amplifier are represented as being -pentode tubes. The cathodes of the two tubes are connected together and the anode 2 of V1 is connected through a blocking condenser C5 to the control grid-5 of tube V2. From the anode 2 of V1 to cathode thereof there is connected a multi-element coupling network which may conveniently be thought of as constituting a single two-terminal .impedance coupling. This network is intended and designed to present across the output terminals of V1 a pure resistance that is of the-samelmagnitude at every frequency within the band occupied by the signals. Across the input terminals of V2 is connected agrid leak comprising resistors R and Re. which dissipate the static charge that tends to accumulate on control grid 5. As thus described the amplifier stages will be understood to be essentially impedance-condenser coupled.

From an intermediate point a of the coupling network a second blocking condenser Cs is connected to an intermediate point of the grid leak, viz., the junction of resistors R5 and Re. Between the point a and the anode 2 the coupling network may be considered to be divided into two parallel branches. One of these branches consists of the inductance L, and the other comprises an artificial line I2 that is made up of lumped inductive and capacitive elements configured and proportioned with relation to the total stray capacitance Co across the coupling circuit as an M-derived 10W-pass filter terminated in a resistor II. The line I2 presents at itsinput terminals, as indicated, a resistance R1 that is constant, or substantially so, excepting possibly at the extreme top portion of the signal frequency range. As will be presently explained in greater detail', the inductance L is so proportionedV in relation to the other circuit constants that itconstitutes almosty a short circuit for frequencies in the lower portion of the signal band sothat across the terminals ofthe portion of the couplingv -circuit described there is developed 'principally only voltages corresponding to frequencies in the upper portion of the signal range. The effect of inductance L may be achieved alternatively and preferably by shunting an inductor across resistor II where its distributed capacitance will not contribute to Cn, but for simplicity of exposition it is shown connected as above described.

Between point a andthe cathode 3 the coupling network is divided into two parallel branches, one a condenser C1 and the other a resistor R2. Although the lower` end of resistor R2 is connected to the cathode 3 only indirectly through the anode biasing source, the latter is presumed to Yhave such low impedance, as by virtue of a large by-pass condenser across its terminals, as to be negligible at all signal frequencies, so that the lower terminal of resistor R2 may be considered to be connected directly to the cathode in so far as the signal frequencies are concerned. As will be presently explained, condenser C1 is so proportioned as to present almost negligible impedance to frequencies in the upper portion of the signal band but substantial impedance to lower vfrequencies so that between point a and cathode 3 the signal voltage developed is due principally to signal currents in the lower portion ofthe signal band.

The unavoidable stray capacitance which, as explained hereinbefore, tends to appear across thecoupling impedance is represented by capitance Co. This capacitance is significant only at high frequencies, and at such frequencies that capacitance Yis virtually connected betweenV anode 2 and point a inasmuch as the condenser C1 is so proportioned asto have small, if Ynot negligible, impedance at high frequencies. Capacitance Cn, then, may be considered as comprising one element of artificial line I2; indeed, it is the very presence of the lcapacitance C0 `that gives rise to the necessity for resorting to an artificial line rather than a simple resistor, remembering that one objective is to maintain the coupling impedance virtually purely resistive. Y'For analytical purposes the combination of Coand the artificial line may be replaced with a non-reactive resistance R1.

The interstage coupling circuit shown in Fig. 1 may now be understood to provide two. paths for the more or less exclusive transmission of different portions of the. frequency range. For high frequency currents, as for example the highest frequency in the signal band, the transmission circuit may be traced from anode 2 through condenser C5 to grid 5 on the one hand, and from cathode 3 to cathode v6 of V2 on the other. The only series element in the circuit traced, viz., blocking condenser C5 is of negligible reactance at the high frequency assumed. At the same time it will be found that every shunt path across the circuit traced has at least one series element of high impedance so that the shunting effect is negligible. Considering now currents in the low frequency portion ofthe signal band, for example the lowest frequency transmitted, a path can be traced from the anode 2 through inductance L,

which is of negligible reactance at the frequency assumed, through blocking condenser Cs which is of large capacitance and through grid leak element R5 to the grid 5. The other side'of the low frequency circuit comprises simply the connection between the two cathodes. It will be noted in this case also that the shunt elements are all of high impedance at the low frequency assumed.

For a better understanding of the circuit features thus far described with reference to Fig. 1`,

reference is now made to Fig. 2 which shows a simplified circuit embodiment. Like elements are represented by the same reference characters in the two figures except that the artificial line I2 and stray capacity C0 have been replaced in Fig. 2 byan electrically equivalent resistor R1. Considering first the lcoupling network in Fig. 2 comprising the `elements L, C1, R1 and R2, it is to `be noted that these four can be so relatively proportioned that` the impedance appearing across the terminals of the coupling network is constant at all frequencies. To produce this constant resistance condition it is necessary only that the following relations obtain:

R1=R2=R 1) and K Blocking condenser C5 may be of fairly small capacitance inasmuch as it is not traversed by low frequency currents. Since it is small it can be designed to have very little stray capacitance to ground. Condenser Ce on the other hand is of large capacitance compared with condenser C5 and it would tend to have proportionately greater stray capacitance which would effectively appear in parallel with condenser C1 and virtually forma part ofthe latter. Blocking condenser Cs is so proportioned with respect to resistor Rs as to accommodate the low frequencies transmitted. Preferably, 21rfC5R5 should be small compared with unity for all frequencies f lying between the top of the signal band and a frequency such that 21rfL is much less than R1. If in a particular case resistors'Rs and Re are not of negligibly high resistance they may be treated as being in parallel with R1 and R2 respectively and as correspondingly modifying the values of R1 and R2 in Equations 1 and 2.

Reverting now to Fig. 1, it will be noted that there is a connection from the screen grid of the pentode V1 to the point a and that biasing voltage for the electrode passes to this point from the anode biasing terminal +B through resistor R2. Biasing voltage for Vthe anode 2 is likewise derived from the point a through inductance L. Inasmuch as the direct current impedance of inductanceL may be negligible, it is evident that anode 2 and screengrid 4 can be, and preferably are, operated at substantially the same positive potential, and high gain `thereby achieved.- At the same time it is to be noted that for low signal frequencies the screen grid 4 and anode 2 are virtually connected together so that the high gain effect a triode has `for low frequencies can be realized. As hereinbefore explained, the coupling network can be proportioned in accordance with Equations 1 and 2 in such manner that its terminal impedance is purely resistive over the entire signal frequency range. Condenser C1 therefore serves, in so far as the featuredescribed in this paragraph is concerned, to restore 1 the resistive character of the coupling impedance 1 that would be lost were only inductance L added.

The necessary modificationis defined by `the following equations: 1

RFGMJVG... (3)

and

where Gmp is the mutual conductance of tube V1 as measured from the control grid to the anode 2 and Gms is the mutual conductance as measured from'the control grid to the screen grid 4. In using these` two equations the internal cathodeanode resistance can be neglected but the internal screen to cathode resistanceshould be in'- cluded in parallel with and treated as a part `of R2." The amplification as measured by the ratio 1 ofthe voltage e2 appearing across the coupling network to 'the voltage e1 applied to the input terminals of tube V1 is then constant at all frequencies and of a magnitude expressed by the following equation:V

It will be evident from the foregoing description of Figs. l and 2 that the inductance L performs multiple functions, one being to provide la metallic connection between the anode 2 and screen grid 4 so that equality of biasing voltage obtains, another that of providing a frequency selective impedance operative to direct the high frequency signal currents to the high frequency portion of the coupling circuit, and another that of providing a low impedance connection from the anode 2 to one side of the low frequency portion of the coupling circuit. Condenser C1 likewise has manifold functions for it preserves the resistive or constant gain character of the coupling network despite the presence of the inductive element L, it distinguishes between the high frequency and low frequency currents in such manner as to direct the `low frequency currents to the low frequency portion of the coupling cir- `introduce in the coupling networkr certain circuit elements properly proportioned to compensate for the gain-modifying characteristics of the biasing circuit. An illustrative example of this phase of the invention is shown in Fig. 3.

Fig. 3 is essentially the same as Fig. 1, or it may be, excepting for the insertion of a resistance R3 in parallel with a condenser Ca in the external lead from cathode 3 of V1 and excepting for the interposition of a like combination, R4 and C21, between the coupling circuit elements C1 and Rz and ground. The resistor Re, inasmuch as itis disposed in the common cathode portion of the anode and control grid circuits of the tube V1, is traversed by the direct current flowing from the anode voltage source and the potential drop across it is impressed as a bias on the control grid. To suppress uctuatio'n in the grid bias so derived the resistor Rs is shunted byrcondenser C3. It is .impractical to have the capacitance C3 so large that; its reactance is negligible and its shunting effect substantially perfect at the exceedingly low frequencies involved in television transmission, hence there is a certain amount of impedance to signal currents of low frequency present in the common cathode portion of the circuit. The impedance is negligible at high frequencies but at frequencies in the lower portion of the signal band it may be large enough to introduce a substantial gain-reducing feedback and associated phase shift so that the gain characteristic of the amplifier tends to A.droop at the lower end of the frequency range; To compensate for the effect described applicant providesin Fig. 3 a resistor Ri and a condenser C4 connected in parallel with each other and disposed in series between the junction of C1 and R2 and ground. Inasmuch as the effect now under consideration is signincant only at the lower end of the frequency range where the coupling impedance of Fig. l effectively degenerates to the resistor R2 alone, the proper proportioning of the significant circuit elements can be described with reference to a simplied circuit such as shown in Fig. 4.

In Fig. 4 the coupling resistance is designated R2, for the reason just stated, and the compensating circuit is connected between its low potential end andground. Actually, of course, R4 is grounded through the negligible impedance of the anode biasing source and not directly.

For thevoltage amplificationez/ei to be constant at all frequencies the several resistors should be proportioned in accordance with the relation:

R4=GmRaR3 (G) where Gm is the effective mutual conductance at the frequencies of interest; and the time constant of the biasing circuit should be equal to that of the compensating circuit, i. e.,

Although these last two equations specify the rel' 'ative proportions of the circuit elements the absolute magnitude of the several impedance elements should be judiciously selected with the possible effect of these elements on other portions and functionsof the circuit in View. Thus in the circuits shown in Fig. 3 the capacitance Cishould preferably be large compared With the stray capacitance to groundof condenser Cs.

Fig. 5 shows` a permissible modification of the Fig. 3 circuit in which the compensating unit Cri-R4 is interposed, as before, between the low potential end of resistance Rz and ground but in which condenser C1 is connected directly to ground. The circuit elements may be proportionedjin accordance with Equations 6 and 7, but in this case capacitance C4 should be large compared with capacitance Ci so that the impedance in series with Rz is small or negligible at the lower end'of lthe signal frequency band.

Quantitative data relevant to a specific amplifier vembodying certain features of the invention may contribute to a better understanding of the invention and the foregoing exposition thereof. The specific amplifier circuit configuration may be assumed to be essentially equivalent to that shown in Fig. 2. It may be further assumed that the amplifier employs screen grid tubes and that it is designed for television signals ranging in frequency from 40 ycycles per second to three meaacycles.

In such. case the circuit elements may havel the following absolute Values: L, 10 millihenries; R1 and R2, 7000 ohms; Ci, 200 mlcro-.microfarads; C5, 500 micro-microfarads; Cs. 0.5 microfarad; and Rsand Re, 50,000 and 250,000 ohms, respectively.` It will be `understood that these data areV only illustrative, not necessarily typicaLland no1'l intended aS, limiting the invention in any degree to the `values specified.

To those skilledin the art it will be apparent that-theinventionandits Various features are susceptible of embodiment in a Wide Variety of forms Within the, spirit and scope of the appended claims.

What is claimed is:

1. A multistage `amplifier comprising a first discharge device having a cathode and anode, a second discharge device having a cathode and control electrode, and an interstage circuit coupling said devices in tandem for the amplification of signals occupying a wide frequency range, said coupling circuit comprising two resistances connected in series` across said cathode and'anode of said first discharge device, an inductance veffectively connected acrossthe one of saidrresistances that is ynearer said anode and effectively shuntingl said resistance only for frequencies in the lower portion of said `frequency range, a capacitance effectively Shunting the other of said resistances only for frequencies in the up per portion of said frequency range, a high resistance connecting said cathode and control electrode `of said second discharge device, a blockingA capacitance connecting said anode and said controllelectrode, saidblocking capacitance having lowreactance for` said frequencies in the upper portions of said frequency range, a second blocking capacitance connecting the junction oi said two resistances andy anlintermediate pointl of said high resistance, said second blocking capacitance having ,relatively low reactance for said frequencies in theloweruportion of ysaid fre- Cluelly fange.-

2, A combination in accordance with claim l in which said tivo resistances, said inductance, and saidnuuting capacitance, are of such relative kmagnitudes that the impedance presented by themtosaidlirst discharge device is substantially thesame.atallfrequencies Awithin said frequency range.

3. In va signal amplifier comprising space dischargearnplifying devices in a plurality of impedance coupled stages, a coupling impedance between two,of said stages consisting essentially of two substantiallyjequal resistancesR connected in series` with each other, aninductance L effectively shunting oneof said resistances, vand a capacitance C effectively shunting the other of said resistances, said elements being proportioned in conformity with the relation L/C=R2, andl means for transferring signal potentials from said coupling impedance to the second of said two stages comprising a coupling connection from the junction of said resistances to said second stage. i

4. In an impedance coupled multistage signal amplifier, a coupling impedance consisting essentially of two substantially equal resistances R connected in series with each other, an inductance, L effectively shunting one of said resistances, and a capacitance C effectively shunting the, other of saidresistances, said elements` being proportioned in conformity with the relation L/ C=R2, the second of the stages coupled by said coupling impedance comprising a space discharge device having an input circuit, and circuitrrl'eans for separately applying to said input circuit the signal voltage appearing across said inductance and the signal voltage appearing across said capacitance.

5. A multistage amplifier for amplifying, with substantially uniform gain, signals occupying at least several octaves in the frequency spectrum, comprising an amplifying space discharge device having a cathode, an anode and a screening grid for said anode, a coupling impedance connected between said cathode and anode comprising two resistances R1 and R2 in series with each other with R1 connected to said anode, an inductance L effectively shunting resistance R1, a capacitance C effectively shunting resistance R2, and a direct connection from said screening grid to the junction of said resistances, the aforementioned elements comprising said coupling impedance being proportioned in conformity with the relation L/C=R1R2 and so that the gain of said space discharge device is substantially the same at all frequencies occupied by said signals.

6. In a multistage signal amplifier comprising a first and a second amplifying discharge device in successive stages thereof, said rst discharge device having a signal output circuit and said second discharge device having a control electrode and a signal input circuit connected thereto, a pair of blocking condensers for said control electrode, one of said condensers having a capacitance many times greater than the other, frequency selective means for applying signals lying principally in the upper portion of the frequency range occupied by said signals from said output circuit to the smaller of said condensers for application to said control electrode, frequency selective means `for applying signals lying principally in the lower portion of said frequency range from said output circuit to the larger of said condensers for application to said control electrode, and an impedance of large magnitude interposed in said input circuit between said larger condenser and said control electrode.

'7. A combination in accordance with claim 6 comprising means for maintaining said larger condenser at substantially ground potential for signal frequencies in the upper portion of said range, whereby the stray capacitance of said larger condenser is effectively shunted across only the low frequency portion of the signaling circuit.

8. In an amplifier for signals occupying a frequency range comprising many octaves, a signal amplifying space discharge device comprising a cathode and a control grid, a grid leak connected in circuit between said cathode and control grid,

a signal input connectionto the said grid comrange, a second signal input connection to an intermediate point on said grid leak comprising a series blocking condenser having negligible series reactance at said lower frequencies, frequency selective means for applying to said rst signal input connection signal frequencies lying principally in the upper portion of said range, frequency selective means for applying .to said second input connection signal frequencies lying principally in the lowerportion of said range, and a third signal input connection to said cathode.

9. A combination in accordance with claim 8 including means for maintaining said blocking condenser in said second input connection at substantially ground potential for frequencies in the upper portion of said range.

1U. A combination in accordance with claim 8 including means for applying said signals to said rst and second input connections and means for maintaining said blocking condensers at substantially the same potential for frequencies in the lower portion of said range.

11. A combination in accordance with claim 8 comprising reactance means shunted between said first and second input connections, said reactance means having small impedance at a frequency in the lower portion of said range.

12. A signal amplifier comprising a space discharge device having a cathode and a control electrode, a high impedance element serially connecting said cathode and control electrode, signal input means for applying signal frequencies lying in the upper portion of the signaling frequency band across one portion of said impedance element, signal input means for applying signal frequencies lying in the lower portion of said band across another portion of said impedance element, and means for maintaining the low potential end of said one portion at substantially ground potential for signal frequencies in said upper portion of the band. Y

13. An amplifier comprising at least two amplifying stages and an interstage circuit coupling said stages, an impedance element shunted across the input of the second of said stages, said interstage circuit comprising a shunt branch divided into two series-connected impedance elements, and at least three signal transferring series branches extending from the several terminals of said series-connected impedance elements to the said input impedance element of the second of said stages, said series-connected impedance elements having different impedance-frequency characteristics such that low frequency signal voltages appear principally across `one of said elements and high frequency signal voltages across the other, and means for maintaining the junction of said impedance elements at substantially ground potential for a frequency lying above said low frequency.

JULIAN M. WEST. 

